1. Field of the Invention
This invention relates generally to systems for the reproduction of recorded information, such as video information recorded on a disc, and, more particularly, to systems for focusing a light beam on an information track on such a disc.
Video and other types of information can be stored on a disc as a succession of light reflective and non-reflective regions along a spiral information track on the surface of disc-shaped record carrier. In reproducing the video information, a video disc player employs an optical system for directing a radiant reading beam, such as a laser beam, onto the information track, and for detecting a reflected beam indicative of the reflectivity of the disc surface scanned by the laser beam as the disc is rotated. In a disc player of the type with which the invention may be employed the intensity of the reflected beam takes the form of a high frequency carrier signal which is frequency modulated by the video information.
In many prior art video disc players, the light beam is focused on a micron wide information track of the video disc. Such a focus often requires a focusing capability of a light spot of less than one micron in width. Slight variations in the distance between the objective lens and the video disc information track can cause a loss of information in the reflected beam at the detection plane. Further undesirable frequencies in the resulting electrical signal can be encountered.
Video discs are subject to surface imperfections which vary the distance between the objective lens focusing the impinging light beam and the information track. Video discs made of plastic, for example, may have varying thicknesses which are unavoidable even in the most sophisticated of replication processes. In disc manufacturing, further, ripple patterns may develop and vertical eccentricities can result from mass manufacturing techniques. Warping from distribution, storage and manufacturing may vary the lens-to-surface distance, especially at the outer edges of the disc where the variations occur at a more rapid rate.
Such variations, if known beforehand, can be accommodated by several techniques. The light source, for example, can be manipulated to re-establish focus. The beam intermediate the light source and the objective lens also can be manipulated. The objective lens itself may be moved toward and away from the video disc along the path of the light. Such movement of the objective lens, however, requires precision apparatus sensing very miniscule spatial movements of the video disc. The lens servo apparatus, moreover, must be capable of determining the correct direction along the reflected light beam axis to insure correct sport focusing on the disc.
2. Description of the Prior Art
Various systems for focusing have been known in the past. Such a system can be seen in the application of James E. Elliott and Lee Mickelson, Ser. No. 909,860, filed May 26, 1978, and now U.S. Pat. No. 4,152,586, issued May 1, 1979, and assigned to the assignee of the present invention which is a continuation-in-part of Ser. No. 803,986, filed June 6, 1977, now abandoned.
The reflected light beam of this system is focused by the objective or focusing lens to a point beyond the lens. Detectors are positioned beyond the focused reflected light beam. A first detector is circular in shape, having its center along the axis of the reflected beam. A concentric second detector has a circular central annulus adapted to surround the first detector while being spaced slightly therefrom. The size of the first detector and of the opening of the second detector is such that approximately equal light intensity is received by each detector when the beam is in optimum focus. The detectors transduce the received light into the electrical signals which are used to activate a servo-system as well as to detect the information on the disc.
This system utilizes focus error signals and tracking error signals at approximately the same frequencies. Therefore, the focus servo system which must be sensitive at the frequencies of the focus errors is also sensitive at the tracking error frequencies. Thus track crossings, track eccentricities, or any other tracking anomalies appear as focus errors.
Accordingly, there has been a need for an improved and more reliable technique of focusing which avoids the use of multiple detectors and lower frequency servo-systems. The present invention is directed principally to this end.